1. Field of the Invention
The present invention relates to an erbium-doped fiber amplifier used in a wavelength division multiplexing (WDM) optical communication system for transmission of optical signals to amplify an input optical signal, and in particular to a wide band erbium-doped fiber amplifier with a gain enhancement.
2. Description of the Related Art
Wavelength division multiplexing (WDM) optical transmission systems are responding to substantial increases in the amount of data transmitted by expanding their transmission bandwidth range. Research efforts involving wide band systems have focused on using both the C-band (Conventional band) and the L-band (Long band). For an optical fiber amplifier adapted to perform an amplification function for optical signals in optical transmission systems, an optical fiber amplifier doped with a rare-earth element, for example, erbium, is widely used. Such an erbium-doped fiber amplifier has a bandwidth limited to about 30 nm for each of the C-band and L-band. Raman amplifiers or tellulite-based erbium-doped fiber amplifiers have a wide amplification bandwidth capable of amplifying both the C-band and the L-band. However, Raman amplifiers have a limited utility because they require a high pumping power and, tellulite-based erbium-doped fiber amplifiers are based on techniques which have not been thoroughly proven. For such tellulite-based erbium-doped fiber amplifiers, research efforts are ongoing.
Current research is being done to develop a wide band erbium-doped fiber amplifier capable of amplifying both the C-band and the L-band by use of a conventional silica-based erbium-doped fiber amplifier. However, most wide band erbium-doped fiber amplifiers have a configuration in which independent C and L-bands are coupled in parallel. FIG. 1 illustrates the configuration of such a prior art wide band erbium-doped fiber amplifier.
As shown in FIG. 1, a typical prior art wide band erbium-doped fiber amplifier includes a first and second parallel-connected fiber amplifier 110 and 120 for amplifying optical signals of the C-band and L-band, respectively. The first fiber amplifier 110 comprises a first erbium-doped fiber (EDF1) 116 adapted to amplify an input optical signal by a pumping light supplied from a 980 nm pumping laser diode 114 connected to the first erbium-doped fiber 116 via a first wavelength selective coupler (WSC) 112. The second fiber amplifier 120 comprises a second erbium-doped fiber (EDF2) 126 adapted to amplify an input optical signal by a pumping light supplied from a forward 980 nm pumping laser diode 124 connected to the second erbium-doped fiber 126 via a second wavelength selective coupler 122, and a pumping light supplied from a backward 1,480 nm pumping laser diode 130 connected to the second erbium-doped fiber 126 via a third wavelength selective coupler 128.
The first erbium-doped fiber 116 amplifies C-band optical signals by a pumping light supplied thereto, whereas the second erbium-doped fiber 126 amplifies L-band optical signals by pumping lights supplied thereto. Upstream from the erbium-doped fibers 116 and 126, a C/L splitter is disposed to split an input optical signal into C and L-band optical signals. Downstream from the erbium-doped fibers 116 and 126, a C/L combiner 134 is disposed to combine the amplified C and L-band optical signals respectively outputted from the erbium-doped fibers 116 and 126. First and second isolators 136 and 138 are also disposed downstream from an input stage, and upstream from an output stage 150, respectively, in order to cut off backward flowing of an amplified spontaneous emission (ASE). The C-band optical signal split by the C/L splitter 132 is subjected to an amplification while passing through the first erbium-doped fiber 116. The amplification is carried out in accordance with an induced emission of excited erbium ions. That is, the erbium ions maintained in a ground state in the first erbium-doped fiber 116 are excited by the pumping light. The excited erbium ions are induced to be emitted, thereby amplifying the C-band optical signal passing through the first erbium-doped fiber 116. Similarly, the L-band optical signal split by the C/L splitter 132 is amplified while passing through the second erbium-doped optical fiber 126.
As discussed above, a typical prior art wide band erbium-doped fiber amplifier is configured by individually preparing C and L-band erbium-doped fiber amplifiers, and connecting those C and L-band erbium-doped fiber amplifiers in parallel. For this reason, a large number of optical elements are required. That is, the prior art wide band fiber amplifier has a disadvantage in terms of the manufacturing cost and decreased reliability due to the number of components involved. Furthermore, an increase in noise occurs in the fiber amplifier because the C/L splitter exhibiting a high insertion loss of about 0.7 dB is disposed upstream from the first and second erbium-doped fibers 116 and 126, each of which is a gain medium.